A System Area Network (SAN) is used to interconnect nodes within a distributed computer system, such as a cluster. The SAN is a type of network that provides high bandwidth, low latency communication with a very low error rate. SANs often utilize fault-tolerant technology to assure high availability. The performance of a SAN resembles a memory subsystem more than a traditional local area network (LAN).
The preferred embodiments will be described implemented in the ServerNet architecture, manufactured by the assignee of the present invention, which is a layered transport protocol for a System Area Network (SAN). The ServerNet II protocol layers for an end node and for a routing node are illustrated in FIG. 1. A single session layer may support one or two ports, each with its associated transaction, packet, link-level, MAC (media access) and physical layer. Similarly, routing nodes with a common routing layer may support multiple ports, each with its associated link-level, MAC and physical layer.
Support for two ports enables ServerNet SAN to be configured in both non-redundant and redundant (fault tolerant, or FT) SAN configurations as illustrated in FIG. 2 and FIG. 3. On a fault tolerant network, a port of each end node may be connected to each network to provide continued message communication in the event of failure of one of the SANs. In the fault tolerant SAN, nodes may be also ported into a single fabric or single ported end nodes may be grouped into pairs to provide duplex FT controllers. The fabric is the collection of routers, switches, connectors, and cables that connects the nodes in a network.
The SAN includes end nodes and routing nodes connected by physical links. Each node may be an end node which generates and consumes data packets. Routing nodes never generate or consume data packets but simply pass the packets along from the source end node to the destination end node.
Each node includes bidirectional ports connected to the physical link. A link layer protocol (LLP) manages the flow of status and packet data between ports on independent nodes.
The ServerNet SAN has been enhanced to improve performance. The original ServerNet configuration is designated SNet I and the improved configuration is designated SNet II. Among the improvements implemented in SNet II SAN is a higher transfer rate and different symbol encoding. Links between SNet II endnodes have a data transfer rate of 125 MB/s. Future CPUs and I/O devices will require much faster data transfer rates. However, to significantly increase the link transfer rate would require discontinuing use of low-cost commoditiy serial links such as the 1.25 Gbit serial links common to Ethernet.